An image sensor comprises in general a matrix of rows and columns of pixels in which each pixel includes a light-sensitive element (generally a photodiode) and several control transistors or gates which have different functions such as for example the selection of the pixel with a view to reading out the signal produced by this pixel, the reset of a charge storage node at each integration period, the reset of the photodiode if necessary, and the transfer of charges from the photodiode to the storage node at the end of an integration duration.
The pixels therefore receive various control signals according to a very precise time sequence for each signal in relation to the other signals. The time sequence repeats periodically according to a frame cycle which provides for reading the information indicating the level of light generated in each of the pixels of the matrix over this cycle.
The signals from the pixels are in general read row by row, the pixels in a row all being selected at the same time by a row selection conductor. Selecting a pixel belonging to this row and to a determined column transfers onto a column conductor common to all the pixels of the same column an electrical signal level generated by the light in this pixel. Readout circuits, at the column base, provide for reading out in parallel the levels received by the column conductors. The rows of pixels are addressed successively in order to be read one after the other by these readout circuits. The control signals for these readout circuits are synchronized with the pixel control signals, for example to sample a voltage level on the column conductor after a reset phase of a charge storage node or after a charge transfer phase from the photodiode to the storage node. The generation of these readout circuit control signals therefore follows rules similar to the generation of the internal control signals for pixels.
Of interest here is the generation of control signals for pixels and/or for readout circuits.
An image sensor can operate in at least two different modes which are respectively:
a “Global Shutter” mode, in which the instant of the start of the integration of charges generated by the light is the same for all the pixels and the instant of the end of the integration of charges after an integration duration Ti (adjustable if necessary) is also the same for all the pixels.
an “Electronic Rolling Shutter”, or ERS, mode, in which the integration start instant is defined for each row and is shifted from one row to the next, the integration duration Ti is the same for all the rows, and the integration end instant is therefore shifted from one row to the next.
It is sometimes desired that the same sensor can operate in both the modes above, or even in other modes, and in each mode with variations in the sequencing of the signals. Various control signal sequencing options for pixels and readout circuits must be able to be generated as easily as possible.
In present-day sensors, the pixel control signals and the column base readout circuit control signals are established by means of sequencing circuits specific to each signal. Registers define for each signal or signal portion a start instant and a duration or an end instant, which are expressed with reference to a count of the number of clock pulses of a reference clock. The start instant for the signals linked to the readout of a row can be defined in relation to a zero instant defined by a row sequencer which defines a periodic row cycle. These registers control a state machine which produces the various signals applied to the pixel matrix and to the column base readout circuits.
For certain control signals, two registers for each signal can suffice, containing respectively a start instant and an end instant or a duration of an active level of the signal. For other signals, for example those which include several pulses at the active level over the same cycle, a greater number of registers, for example four or five registers, may be required.
All in all, to create the various signals required for the readout of the pixels, several tens of registers must be provided in the sensor and the registers must be programmed individually.